A new chromosomal sex-determining mechanism inMicrotus arvalis pallas

The karyotype in the rodentMicrotus arvalis comprises 21 autosome pairs and two heterosome pairs of the X₁X₂Y₁Y₂/X₁X₁X₂X₂ type. The occurrence of multiple sex chromosomes is thought to be due to a translocation of one arm of a metacentric autosome to the Y chromosome. This translocation would result in an additional acrocentric sex chromosome confined to the(heterogametic) male line, i.e., a Y₂. The original metacentric chromosome thereby turns into an X₂. Because of the translocation mentioned, a trivalent figure of the Y₁Y₂X₂ type occurs in the first meiotic metaphase in the male.     

Ultrastructural characterization of the sex chromosomes during spermatogenesis of spiders having holocentric chromosomes and a long diffuse stage

An ultrastructural study has been made of spermatogenesis in two species of primitive spiders having holocentric chromosomes (Dysdera crocata, XO and Segestria florentia X₁X₂O). Analysis of the meiotic prophase shows a scarcity or absence of typical leptotene to pachytene stages. Only in D. crocata have synaptonemal complex (SC) remnants been seen, and these occurred in nuclei with an extreme chromatin decondensation. In both species typical early prophase stages have been replaced by nuclei lacking SC and with their chromatin almost completely decondensed, constituting a long and well-defined diffuse stage. Only nucleoli and the condensed sex chromosomes can be identified. — In S. florentina paired non-homologous sex chromosomes lack a junction lamina and thus clearly differ from the sex chromosomes of more evolved spiders with an X₁X₂O male sex determination mechanism. In the same species, sex chromosomes can be recognized during metaphase I due to their special structural details, while in D. crocata the X chromosome is not distinguishable from the autosomes at this stage. — The diffuse stage and particularly the structural characteristics of the sex chromosomes during meiotic prophase are reviewed and discussed in relation to the meiotic process in other arachnid groups.     

Multiple sex chromosome system of X1X1X2X2/X1X2)Y type in lutjanid fish, Lutjanus quinquelineatus (Perciformes)

The karyotype and other chromosomal markers as revealed by C-banding and Ag-staining were studied in Lutjanus quinquelineatus and L. kasmira (Lutjanidae, Perciformes). While in latter species, the karyotype was invariably composed of 48 acrocentric chromosomes in both sexes, in L. quinquelineatus the female karyotype had exclusively 48 acrocentric chromosomes (2n = 48) but that of the male consisted of one large metacentric and 46 acrocentric chromosomes (2n = 47). The chromosomes in the first meiotic division in males showed 22 bivalents and one trivalent, which was formed by an end-to-end association and a chiasmatic association. Multiple sex chromosome system of X₁X₁X₂X₂/X₁X₂Y type resulting from single Robertsonian fusion between the original Y chromosome and an autosome was hypothesized to produce neo-Y sex chromosome. The multiple sex chromosome system of L. quinquelineatus appears to be at the early stage of the differentiation. The positive C-banded heterochromatin was situated exclusively in centromeric regions of all chromosomes in both species. Similarly, nucleolus organizer region sites were identified in the pericentromeric region of one middle-sized pair of chromosomes in both species. The cellular DNA contents were the same (3.3 pg) between the sexes and among this species and related species.     

THE CONTRIBUTION OF FEMALE MEIOTIC DRIVE TO THE EVOLUTION OF NEO‐SEX CHROMOSOMES

Sex chromosomes undergo rapid turnover in certain taxonomic groups. One of the mechanisms of sex chromosome turnover involves fusions between sex chromosomes and autosomes. Sexual antagonism, heterozygote advantage, and genetic drift have been proposed as the drivers for the fixation of this evolutionary event. However, all empirical patterns of the prevalence of multiple sex chromosome systems across different taxa cannot be simply explained by these three mechanisms. In this study, we propose that female meiotic drive may contribute to the evolution of neo‐sex chromosomes. The results of this study showed that in mammals, the XY₁Y₂ sex chromosome system is more prevalent in species with karyotypes of more biarmed chromosomes, whereas the X₁X₂Y sex chromosome system is more prevalent in species with predominantly acrocentric chromosomes. In species where biarmed chromosomes are favored by female meiotic drive, X‐autosome fusions (XY₁Y₂ sex chromosome system) will be also favored by female meiotic drive. In contrast, in species with more acrocentric chromosomes, Y‐autosome fusions (X₁X₂Y sex chromosome system) will be favored just because of the biased mutation rate toward chromosomal fusions. Further consideration should be given to female meiotic drive as a mechanism in the fixation of neo‐sex chromosomes.     

Pairing and segregation of the sex chromosomes in X1X2-males of Dysdercus intermedius with a note on the kinetic organization of Heteropteran chromosomes

The existence of an X₁X₂-mode of sex determination is confirmed by a study of all meiotic stages in the male cotton stainer (X₁X₂ and pertinent stages in the female (X₁X₁ X₂X₂). In the male, the X-chromosomes are heterochromatic and pair end-to-end in early meiotic prophase. At diakinesis, they disjoin and align side-by-side in the center of the spindle, forming a pseudotetrad. Anaphase I is equational for the sex chromosomes. At late anaphase or telophase, X₁ and X₂ join end-to-end but form spindle fiber connections to only one of the poles of the metaphase II spindle, leading to one daughter cell without X chromosomes and one with both X₁ and X₂. An attempt is made to explain sex chromosome pairing and orientation on the basis of a telocentric organization of meiotic chromosomes. The apparent differences in the kinetic organization of mitotic and meiotic chromosomes in Heteroptera are discussed.     

Conserved autonomy of replication of the X-chromosomes in hybrids of Drosophila miranda and Drosophila persimilis

The chromosome complement of hybrid males from the cross between Drosophila miranda female and D. persimilis male provides an interesting chromosomal situation where an autosome, the 3rd chromosome of D. persimilis, coexists with a homologue that developed into a sex chromosome, the X₂ in D. miranda. Except for certain inversions and a few minor translocations, these two chromosomes (X₂ and the 3rd) still look alike as polytene elements. However, in hybrid males pairing of the two chromosomes, the X₂ and 3rd, is rare, while in female hybrids it occurs frequently. — ³H-TdR labeling shows that while the X₂ and 3rd chromosomes replicate synchronously in hybrid female, in the hybrid male the former completes its replication earlier than the 3rd chromosome, as do the two arms of the X₁ (XL and XR). The frequency and relative intensity of ³H-TdR labeling of each site of the X₂ and that of the 3rd chromosome in hybrid males closely agree with those of the corresponding sites in the X₂ of the miranda male and the 3rd chromosome of the persimilis male (or female), respectively. The results suggest that timing and rate of replication of the X₂ are determined autonomously and follow the pattern in the respective parental species.     

Cytogenetic analysis of Ctenostomini by C-banding and rDNA localization and its relevance to the knowledge of the evolution of tiger beetles (Coleoptera: Cicindelidae)

In this work, the first cytogenetic data on Neotropical Collyrinae is provided, by way of their karyotypes, C-banding and ribosomal genes (rDNA) localization using fluorescence in situ hybridization (FISH). The two species analysed, Ctenostoma (Procephalus) ornatum ornatum (male) and Ctenostoma (Euctenostoma) rugosum(female) showed, respectively, a diploid number of 17 and 18 chromosomes. C. ornatum ornatum has a multiple sex chromosome system ( n=7 + X₁X₂Y), and mitotic and meiotic metaphase cells showed rDNA gene labelling in the smallest autosomal pair. In this species, no C-bands were obtained, while C. rugosum seems to exhibit centromeric and/or interstitial C-bands in almost all chromosomes. The observation of a multiple sex chromosome system in Ctenostomini ensured the appearance of this characteristic in the hypothetical ancestral of Collyrinae and Cicindelini. The subfamily Collyrinae is not uniform in what concerns diploid chromosome number and rDNA gene localization, because C. ornatum ornatum possesses a lower chromosome number and autosomal rDNA genes when compared with the other Collyrinae species studied ( Neocollyris spp.). Independent events leading to the reduction in chromosome number might have taken place during the split of the Collyrinae into the tribes Ctenostomini and Collyrini.     

Cytotaxonomy of the triatominae (Reduviidae: Hemiptera)

The chromosome number and meiotic cycle of 20 species of Triatominae have been investigated. In the male, there are five types of chromosome complement: 20+XY, 20+X₁X₂Y, 20+X₁X₂X₃Y, 18+XY and 22+XY.The cytological data suggest that the type number for the subfamily is 22 (20+XY). In the hybrids: Triatoma barberi () and T. protracta (), anomalous behavior of certain chromosomes has been observed. Phylogenetic relationships based on chromosome evidence in the subfamily have been discussed. It is suggested that fragmentation is the major factor for chromosome evolution in the group.     

An ultrastructural cytogenetic study on the evolution of sex chromosomes during the spermatogenesis of Lycosa malitiosa (Arachnida)

In an electron microscopic study concerning the structure of sex chromosomes of the spider Lycosa malitiosa (male 2n=20 X₁ X₂O) and their behaviour during spermatogenesis, based on a detailed analysis of meiotic stages (including tridimensional reconstructions), we established: a) the existence of a special type of pairing between non-homologous sex chromosomes, which become associated by a particular laminar structure; b) the existence of a connection between both sex chromosomes and two or three autosomes. This connection is mediated by well-defined structures located in small regions of the sex chromosome surface; c) the presence of short fibrillar axes in the middle of each sex chromosome. — The scarce literature on the ultrastructure of invertebrate sex chromosomes is commented upon and the probable functional interpretation of the described structures is discussed.This paper is dedicated to the memory of the late professor Francisco Alberto Saez, whose permanent interest stimulated in our Division the development of cytogenetical studies at the ultrastructural level     

X1X1X2X2/X1X2Y sex chromosome systems in the Neotropical Gymnotiformes electric fish of the genus Brachyhypopomus

Several types of sex chromosome systems have been recorded among Gymnotiformes, including male and female heterogamety, simple and multiple sex chromosomes, and different mechanisms of origin and evolution. The X₁X₁X₂X₂/X₁X₂Y systems identified in three species of this order are considered homoplasic for the group. In the genus Brachyhypopomus, only B. gauderio presented this type of system. Herein we describe the karyotypes of Brachyhypopomus pinnicaudatus and B. n. sp. FLAV, which have an X₁X₁X₂X₂/X₁X₂Y sex chromosome system that evolved via fusion between an autosome and the Y chromosome. The morphology of the chromosomes and the meiotic pairing suggest that the sex chromosomes of B. gauderio and B. pinnicaudatus have a common origin, whereas in B . n. sp. FLAV the sex chromosome system evolved independently. However, we cannot discard the possibility of common origin followed by distinct processes of differentiation. The identification of two new karyotypes with an X₁X₁X₂X₂/X₁X₂Y sex chromosome system in Gymnotiformes makes it the most common among the karyotyped species of the group. Comparisons of these karyotypes and the evolutionary history of the taxa indicate independent origins for their sex chromosomes systems. The recurrent emergence of the X₁X₁X₂X₂/X₁X₂Y system may represent sex chromosomes turnover events in Gymnotiformes.     

A complex sex-chromosome system in the hare-wallaby Lagorchestes conspicillatus Gould

Among specimens of the spectacled hare-wallaby Lagorchestes conspicillatus Gould (Marsupialia, family Macropodidae) 4 males had 15 chromosomes and 2 females 16 chromosomes. The sex chromosomes are X₁X₁X₂X₂ in the female and X₁X₂Y in the male, the Y being metacentric and both X chromosomes are acrocentric. In about 96% of sperm mother cells at meiosis the sex chromosomes form a chain trivalent and in more than 99% of these this orients convergently so that the X₁ and X₂ move to the same pole. Evidence is presented that L. conspicillatus has evolved from a form with 22 chromosomes including a small X and a minute Y. Autoradiographic studies show that the proximal fifth of the X₁ chromosome replicates late. This is probably the ancestral X chromosome which has been translocated to an autosome. The fate of the original Y is obscure but an hypothesis is proposed that it forms the centromeric region of the Y. A single male had 14 chromosomes and was heterozygous for a translocation involving the centric fusion of two acrocentric autosomes. In about 30% of sperm mother cells the autosomal trivalent did not disjoin regularly but, despite this, all secondary spermatocytes observed at metaphase 2 had balanced complements of chromosomes. It is assumed that unbalanced secondary spermatocytes died before reaching metaphase.     

Gene mapping studies confirm the homology between the platypus X and echidna X1 chromosomes and identify a conserved ancestral monotreme X chromosome

The identification of the sex chromosomes in the three extant species of Prototherian mammals (the monotremes) is complicated by their involvement in a multivalent translocation chain at the first division of male meiosis. The platypus X chromosome, identified by the presence of two copies in females and one in males, has been found to possess a suite of genes that have been mapped to the X chromosomes of all eutherian and metatherian mammals. We have extended gene mapping studies to a member of the only other extant monotreme family, the echidna, which has a G-band equivalent X₁ chromosome, as well as a smaller X₂. We find that the five human X-linked genes (G6PD, GDX, F9, AR and MCF2) map to the echidna X₁ chromosome in locations equivalent to those on the platypus X. These results confirm that the echidna X₁ is the original X chromosome in this species, and identify a conserved ancestral monotreme X chromosome.     

Stability of a multiple X chromosome system and associated B chromosomes in birch aphids (Euceraphis spp.; Homoptera: Aphididae)

Autosomal dissociations are a common feature of aphid karyotype evolution, but multiple X chromosome systems are rare. Birch-feeding aphids of the genus Euceraphis, however, have X₁X₂O males as a general rule, X₁ being always much larger than X₂. Only one species has XO males, and this condition appears to be secondary. Most Euceraphis karyotypes also have one or more, usually heterochromatic, elements that occur in the same numbers in both males and females, yet behave like X chromosomes at male and female meiosis I. They appear to be supernumerary, non-functional X chromosomes, although showing greater within-species stability in size and number than typical B chromosomes. Euceraphis gillettei forms a separate group within the genus and feeds on alders (Alnus species), yet has a similar system, and the two most closely related genera, Symydobius and Clethrobius, also have additional chromosomal elements possibly representing non-functional X chromosomes. Thus the multiple X chromosome system in these aphids seems to be a primitive condition.     

Evolution of karyotype,sex chromosomes,and meiosis in mygalomorph spiders (Araneae: Mygalomorphae)

Spider diversity is partitioned into three primary clades, namely Mesothelae, Mygalomorphae, and Araneomorphae. Mygalomorph cytogenetics is largely unknown. Our study revealed a remarkable karyotype diversity of mygalomorphs. Unlike araneomorphs, they show no general trend towards a decrease of 2n, as the chromosome number was reduced in some lineages and increased in others. A biarmed karyotype is a symplesiomorphy of mygalomorphs and araneomorphs. Male meiosis of some mygalomorphs is achiasmatic, or includes the diffuse stage. The sex chromosome system X₁X₂0, which is supposedly ancestral in spiders, is uncommon in mygalomorphs. Many mygalomorphs exhibit more than two (and up to 13) X chromosomes in males. The evolution of X chromosomes proceeded via the duplication of chromosomes, fissions, X–X, and X‐autosome fusions. Spiders also exhibit a homomorphic sex chromosome pair. In the germline of mygalomorph males these chromosomes are often deactivated; their deactivation and pairing is initiated already at spermatogonia. Remarkably, pairing of sex chromosomes in mygalomorph females is also initiated at gonial cells. Some mygalomorphs have two sex chromosome pairs. The second pair presumably arose in early‐diverging mygalomorphs, probably via genome duplication. The unique behaviour of spider sex chromosomes in the germline may promote meiotic pairing of homologous sex chromosomes and structural differentiation of their duplicates, as well as the establishment of polyploid genomes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 377–408.     

The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z

Background

Sex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.

Results

Chromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X₁, X₂, X₃, X₅, and in chromosome Y₁.

Conclusion

Monotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.     

Supernumerary chromosomes in the human bed bug,Cimex lectularius Linn. (Cimicidae:Hemiptera)

The chromosome complement in the human bed bug, Cimex lectularius Linn., is 26+X₁X₂Y in the male and 26+X₁X₁X₂X₂ in the female. However, a population from Cairo, Egypt has 4 supernumerary X chromosomes. In the hybrid between the Berkeley population (with no supernumeraries) and the Cairo population (with 4 supernumeraries), the behavior of supernumeraries was observed during embryogenesis and oogenesis as well as spermatogenesis.In embryogenesis the transmission of supernumeraries was quite regular. However, one chromosome may sometimes be eliminated early in the germ line. This abnormality could induce the variations in chromosome number encountered in later stages. In the first meiotic division, some of the supernumeraries were nondisjunctional. Moreover, in the second division, some supernumeraries were eliminated. These results show that there is a tendency towards a decrease in the number of supernumeraries in the hybrids.Although the supernumeraries behave like X chromosomes, they seem not to be important for sex determination and appear to be largely or entirely inert genetically. Supernumeraries in the bed bug originate from small fragments caused by structural rearrangement. They are increased by an accumulation mechanism. Supernumeraries in the bed bug appear to be of relatively recent origin. The phylogenetic sequence in their development was probably from none to a stabilized number of four in the Old World. Then the supernumeraries were lost in two specialized lines, Cimex columbarius Jenyns on domestic birds in Europe and Cimex lectularius Linn. on man in the Western Hemisphere.This study was carried out under U.S. Public Health Service Grant (GM-13197).     

Chromosome banding in amphibia

A cytogenetic study performed on a population of the South American leptodactylid frog Eleutherodactylus maussi revealed multiple sex chromosomes of the X₁X₁X₂X₂/X₁X₂Y (=XXAA/XXA^Y) type. The diploid chromosome number is 2n=36 in all females and 2n=35 in most males. The multiple sex chromosomes originated by a centric fusion between the original Y chromosome and a large autosome. In male meiosis the X₁X₂Y (=XXA^Y) multiple sex chromosomes form a classical trivalent configuration. E. maussi is the first species discovered in the class Amphibia that is distinguished by a system of multiple sex chromosomes. Only one single male was found in the population with 2n=36 chromosomes and lacking the Y-autosomal fusion. This karyotype (XYAA) is interpreted as the ancestral condition, preceding the occurrence of the Y-autosome fusion.by H.C. Macgregor     

Chromosome elimination and sex determination in springtails (Insecta, Collembola).

A post-zygotic mechanism of sex determination is described in the two symphypleonans Dicyrtomina ornata (Nicolet) and Ptenothrix italica Dallai. The process consists of the loss of two sex chromosomes from the male embryo. At the end of the first meiotic division of spermatogenesis, a second chromosome elimination occurs, allowing half the secondary spermatocytes, later transformed into spermatids, to receive a complete haploid set of chromosomes. The secondary spermatocytes, which receive an incomplete set of chromosomes, degenerate. Males of the two collembolan species, therefore, produce a reduced number (50%) of spermatozoa. Females of D. ornata have 2n = 12 and males 2n = 10 chromosomes; females of P. italica have 2n = 14 and males 2n = 12 chromosomes. In both species, oogenesis proceeds normally and chromosomes pair and form chiasmata in meiotic prophase. The adaptive significance of this post-zygotic mechanism of sex determination is discussed. The mechanism seems to be a characteristic feature of the suborder Symphypleona. The neanurid Arthropleona Anurida maritima (Guérin), which was studied for comparative analysis, has 2n = 8 chromosomes and normal spermatogenesis producing haploid nuclei with four chromosomes. J. Exp. Zool. (Mol. Dev. Evol.) 285:215-225, 1999.